I cannot stress enough that the GWS Beaver flies wonderfully on the stock brushed motor and prop with a 2 cell Lipo battery and any old 5-10 amp speed control. The stock power plant has even been used to fly the GWS Beaver on floats from water so dont let the information below lead you to think you need to upgrade or go brushless because in all honesty it is absolutely uneccessary.

There are nearly as many power systems are there are GWS Beavers! Here are Just a few that have been successfully used by the members of RC Groups:

My setup (bob_suruncle):
-Motor: Artech 3700 KV Brushless in runner but have also had success with an E-Flite park 370 brushess in-runner as well
-Speed control: Castle creations pixie 20
-Battery: Thunder Power 3S 1320 LiPo or Artech 3s 1800
-Gearing: 5.33:1 or GWS "C" Gearing
Prop: GWS 9070 3 blade originally but now using either a Master Airscrew 8x6 3 or 2 blade prop
-EST Flight times: 15 minutes / charge on the 1320 and 20+ on the 1800

(NOTE: Old #1 still flies reliably on is brushed "C" gearing power system and a 2S Lipo)